For easy carrying and transport, most of the present pet bag racks are foldable. For example, Chinese patent CN101375671A discloses a pet bag rack. It comprises a square underframe. Two inverted U-shaped side frames are hinged symmetrically on the two sides of the square underframe. Upper bars are hinged symmetrically on the two sides of the upper part of a U-shaped side frame, and upper bars are hinged in the corresponding positions of the other U-shaped frame, thus two opposite upper bars may be spliced to form a long bar. A jack is arranged on one of the two opposite upper bars, a notch is arranged on the surface of the upper bar in the section of the jack, a spring is installed inside the jack, a sleeve is disposed on the upper bar with a jack, and a screw hole is arranged on the sleeve, which works together with the notch to realize connection with the corresponding upper bars. The components of this invention won't fall apart after disassembly. Moreover, it achieves the effect of conveniently adjusting its dimensions. Chinese patent CN201199840Y discloses a round-top single-brace pet bag rack, which comprises bottom frame, side frame and brace. On a group of opposite sides of the rectangular bottom frame, ∩-shaped side frames are symmetrically installed through rivets and can revolve around the rivets. The brace is composed of two spliced short bars. The splicing ends of the two short bars are respectively provided with a convex joint and a concave joint. The other ends of the two short bars are respectively connected to the top of the side frames with rivets. A handlebar is installed around the splicing end of one of the above-mentioned short bars. A bolt is disposed on the handlebar and received by the sliding groove of this short bar. In the cavity of the short bar, a spring is provided to withstand the bolt. This round-top single-brace pet bag rack saves material, reduces weight and facilitates use, storage and transport. The above-mentioned two existing pet bag racks are detachable, foldable and facilitate transport and carrying, but as their side frames all adopt an ∩-shaped integral structure, they still have relatively larger volume when folded, even though it has been reduced compared with unfolded, and its transport and carrying are still inconvenient.